It is believed that hydroxyl ion (OH.sup.-) enriched alkaline water, which is often incorrectly referred-to as "alkaline ion water", is useful in health maintenance when served as potable water as well as in accentuating taste when used in cooking or for the preparation of beverages such as tea and coffee. Similarly, hydrogen ion (H.sup.+) enriched acidic water is known as being suitable for boiling noodles and washing the face. More importantly, highly acidic water which is obtained by electrolysis of tap water containing sodium chloride or an aqueous solution of sodium chloride and which therefore contains effective chlorine (hypochlorous acid or chlorine gas) has been noted as having a strong germicidal effect.
To produce alkaline and/or acidic water, an apparatus for electrolyzing water has been used hitherto which is often incorrectly referred-to in the art as an "ion-water generator". This apparatus, designed to subject water to electrolysis, includes an electrolytic cell having an anode and a cathode. As a direct electric potential is applied between the electrodes, the hydroxyl ions OH.sup.- being present in water due to electrolytic dissociation of water molecules will donate electrons to the anode at the anode-water interface and are thereby oxidized to form oxygen gas which is then removed away from the system. As a result, the H.sup.+ concentration is enhanced at the anode-water interface so that H.sup.+ enriched acidic water results at the anode-water interface. At the cathode-water interface, on the other hand, H.sup.+ accepts electrons from the cathode and is reduced to hydrogen to form hydrogen gas which is similarly eliminated from the system. As a result, the OH.sup.- concentration is increased whereby OH.sup.- enriched alkaline water is generated on the cathode side. When an aqueous solution of sodium chloride is subjected to electrolysis, chlorine gas is generated at the anode and is dissolved into water to form hypochlorous acid.
To preclude alkaline water and acidic water once generated by electrolysis from being mixed with each other and to remove them separately, the conventional electrolytic cells are typically provided with a water-impermeable, electrically conducting but ion-permeable membrane 3 arranged between an anode plate 1 and a cathode plate 2 as schematically shown in FIG. 1, the electrolytic chamber being divided by the membrane into a flowpath 4 for alkaline water and a flowpath 5 for acidic water. The electrolytic cell of this type will be referred-to hereinafter as the "membrane-type" electrolytic cell.
As the electrolytic cell is operated, precipitation of scale 6 comprised of calcium carbonate, calcium hydroxide, magnesium hydroxide and the like takes place in the flowpath for alkaline water. Referring to FIG. 2 wherein the apparent solubility of calcium carbonate versus pH is shown, the mechanism of scale precipitation will be described with reference to calcium hydroxide by way of an example. It will be noted from the graph that under acidic conditions, calcium carbonate is dissolved into water in the form of calcium ions. However, as the pH exceeds 8, the solubility rapidly drops so that calcium carbonate settles in the flowpath for alkaline water. The calcium ions are moved toward the cathode plate under the action of the electric field between the electrodes whereby the calcium ion concentration in the flowpath for alkaline water is increased. This in turn promotes precipitation of calcium carbonate.
In the electrolytic cell of the membrane type, the scale tends to precipitate predominantly on the membrane 3 rather than on the cathode 2, as shown in FIG. 1. Probably, this is because the porous nature of the membrane promotes precipitation of the scale, in contrast to the cathode generally having a polished specular surface. Since the precipitates such as calcium carbonate are electrically insulating, the electrical resistance across the cell is increased thereby lowering the efficiency of electrolysis of the cell. In addition, formation of scale increases the flow resistance across the electrolytic cell. Therefore, unless the scale is removed, the electrolytic cell would become inoperative soon after a short period of use.
Accordingly, there has been proposed in the prior art to remove the precipitates by dissolving them into water as disclosed, for example, in Japanese Patent Kokai Publication 51-77584, Japanese Utility Model Kokai Publication 55-91996, Japanese Utility Model Kokai Publication 59-189871, and Japanese Patent Kokai Publication 1-203097. According to this method, a polarity reversal switch 7 (FIG. 1) is turned over in such a manner that an electric potential of an polarity opposite to the normal operating polarity is applied between the electrodes to thereby cause the precipitates to dissolve. This method is known in the art as "reverse electrolysis descaling" or "reverse potential descaling" process. The principle of reverse electrolysis descaling is that, upon application of electric potential of the opposite polarity, the flowpath for the alkaline water is changed into acidic conditions whereby the scale such as calcium carbonate is disintegrated into ions to again dissolve into water as will be understood from FIG. 2.
However, since the membrane 3 is more or less spaced from the electrodes as will be understood from FIG. 1, the stream of strongly acidic water which has been generated along the surface of the electrode 2 (originally acting as the cathode, but now acting as the anode because the polarity of potential is reversed) will be carried away by the flow of water flowing through the flowpath so that strongly acidic water could not reach the membrane as long as it is present in moving water. Therefore, the membrane cannot be rendered acidic to a degree strong enough to quickly dissolve the scale deposited on the membrane. Moreover, where the application of reverse voltage is carried out while flow of water is stopped, the hydrogen ions generated at the surface of the initial cathode 2 (now anode because the potential is reversed) will permeate through the membrane 3 and will be diffused toward the opposite flowpath so that acid water and alkaline water once generated are mixed with each other and are neutralized. As a result, the membrane cannot be rendered strongly acidic. According to the experiment carried out by the present inventors, the pH of the flowpath 4 originally for alkaline water did not become less than 3 when the reversed polarity potential was applied. No removal of scale was observed even after about two days of application of the reversed polarity potential.
In this manner, in the "membrane-type" electrolytic cell, it has been difficult to electrochemically remove the scale even though the so-called reverse electrolysis descaling is carried out. Accordingly, it has been usual that the life of the electrolytic cells is only from a half to one year unless the cells are periodically disassembled and are subjected to manual mechanical descaling operations. Furthermore, the membrane is unhygienic since it serves as breeding bed for bacteria. In addition, the space between the electrode plates must be set large enough to ensure that the membrane is installed therebetween. This leads to the disadvantage that the power consumption of the electrolytic cell becomes large.
In order to overcome the foregoing disadvantages of the membrane-type electrolytic cell, proposed in Japanese Patent Kokai Publication 4-284889 is an electrolytic cell which is free from a membrane. The electrolytic cell of this type will be referred-to hereinafter as the "non-membrane" or "membraneless" type electrolytic cell. In the non-membrane type cell, the electrode plates are spaced from one another with a small gap in such a manner that a laminar flow is established as water flows between the electrodes. Therefore, alkaline water and acidic water as generated can be separated from each other without recourse to a membrane.
As the membraneless-type electrolytic cell is not provided with a membrane which is susceptible to deposition of scale, there is an advantage that less scale is deposited. Moreover, the cell is hygienic because of the absence of a membrane which would otherwise breed bacteria. Furthermore, it is possible to reduce the electrode spacing to thereby reduce the power consumption of the electrolytic cell. The non-membrane type electrolytic cell is also designed such that the reverse polarity potential is applied to carry out the so-called reverse electrolysis descaling in a manner similar to the conventional membrane-type electrolytic cells. Accordingly, it is possible to remove substantially all of the scale that has precipitated on the surface of the cathode plate.
In the non-membrane type electrolytic cell disclosed in JP 4-284889, it is desirable to arrange the cathode and anode with as small spacing as possible with one another to ensure that the electric power consumption is reduced as well as to ensure that a laminar flow is readily established in the flow of water flowing in the flowpath defined between the cathode and anode. In that event, it is then preferable to sandwich between the cathode and anode one or more electrically insulating spacers of a predetermined thickness to ensure that, even if the electrodes undergo a certain degree of distortion or strain, the electrode spacing is kept constant throughout the flowpath to thereby prevent the electrolyzing performance from fluctuating from cell to cell.
However, in the case where the spacers are placed in this manner between the electrodes, the scale tends to precipitate on the lateral surface of respective spacers so that the flowpath of water will be gradually clogged with the scale, as described later with reference to the accompanying drawings. It is believed that this is due to the fact that the scale readily precipitates on the lateral surface of the spacers since the velocity of water flowing along the boundary between the spacers and water is so slow. The scale that has precipitated on the lateral surface of the spacers could not be readily removed by the application of the reverse polarity potential (i.e., the so-called reverse electrolysis descaling process). Once precipitated, the scale gives rise to the formation of turbulent flow so that the flow of water becomes even slower whereby precipitation of the scale is further promoted.
An object of the invention is to improve the non-membrane type electrolytic cell disclosed in JP 4-284889 and to provide a non-membrane type electrolytic cell that is capable of effectively preventing the accumulation of scale caused by the spacers.
In the non-membrane type electrolytic cell, there exists, in the inlet area of the flowpath defined between the cathode and anode, a turbulent flow region wherein a stable laminar flow is not established as yet. In this turbulent flow region, it is not possible to produce highly acidic water along the surface of the electrode plates. Therefore, it is difficult to effectively dissolve the scale into water even if a reverse polarity potential is applied periodically.
Accordingly, another object of the invention is to provide a non-membrane type electrolytic cell which is capable of effectively preventing the accumulation of scale throughout substantially the entire length of the electrodes.